[InstCombine] add tests for rL169025; NFC

This narrow fold was added with no motivation or test cases 
a bit over 5 years ago. Removing a constant operand is a 
good canonicalization? We should handle Y*2.0 too then?

llvm-svn: 326606

1 files changed, 32 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstCombine/fmul.ll b/llvm/test/Transforms/InstCombine/fmul.ll
index b46897c3830..a1c0ae12153 100644
--- a/llvm/test/Transforms/InstCombine/fmul.ll
+++ b/llvm/test/Transforms/InstCombine/fmul.ll
@@ -300,3 +300,35 @@ define float @reassoc_common_operand_multi_use(float %x, float %y) {
   ret float %mul2
 }
 
+declare float @llvm.log2.f32(float)
+
+; X * log2(Y * 0.5) = X * log2(Y) - X
+
+define float @log2half(float %x, float %y) {
+; CHECK-LABEL: @log2half(
+; CHECK-NEXT:    [[LOG2:%.*]] = call fast float @llvm.log2.f32(float [[Y:%.*]])
+; CHECK-NEXT:    [[TMP1:%.*]] = fmul fast float [[LOG2]], [[X:%.*]]
+; CHECK-NEXT:    [[MUL:%.*]] = fsub fast float [[TMP1]], [[X]]
+; CHECK-NEXT:    ret float [[MUL]]
+;
+  %halfy = fmul fast float %y, 0.5
+  %log2 = call fast float @llvm.log2.f32(float %halfy)
+  %mul = fmul fast float %log2, %x
+  ret float %mul
+}
+
+define float @log2half_commute(float %x1, float %y) {
+; CHECK-LABEL: @log2half_commute(
+; CHECK-NEXT:    [[X:%.*]] = fdiv float [[X1:%.*]], 7.000000e+00
+; CHECK-NEXT:    [[LOG2:%.*]] = call fast float @llvm.log2.f32(float [[Y:%.*]])
+; CHECK-NEXT:    [[TMP1:%.*]] = fmul fast float [[X]], [[LOG2]]
+; CHECK-NEXT:    [[MUL:%.*]] = fsub fast float [[TMP1]], [[X]]
+; CHECK-NEXT:    ret float [[MUL]]
+;
+  %x = fdiv float %x1, 7.0 ; thwart complexity-based canonicalization
+  %halfy = fmul fast float %y, 0.5
+  %log2 = call fast float @llvm.log2.f32(float %halfy)
+  %mul = fmul fast float %x, %log2
+  ret float %mul
+}
+